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\section{Introduction}
\label{sec:intro}
The radio access network (RAN) of the cellular network infrastructure provides wide-area 
wireless connectivity to mobile devices. Current LTE RANs consist of a collection of 
largely independent base stations. The data plane of 
base stations runs standard LTE PHY and MAC, and supports a small number of QoS classes. 
Base stations run distributed algorithms to loosely 
coordinate among each other for handover and interference management. 

The current RAN architecture faces three big trends. To cope with exponential traffic 
growth, operators are increasingly deploying small cell base stations in a dense and 
unplanned fashion. To support the growing diversity of applications such as intermittent 
machine type-communication (MTC)~\cite{M2Msigm12} 
%, smart grids
and automotive applications, 
new protocols are being standardized.
To reduce the huge deployment and operational cost, 
LTE RAN infrastructure sharing mechanisms have been developed. 

However, these band-aid solutions run into significant problems. 
First, distributed 
coordination with unplanned and dense deployment makes radio resource allocation very 
inefficient and makes managing interference harder. This calls for a logically centralized 
coordinator. 
Second, current RAN data plane at layer 1 and layer 2 (L1/L2) is not 
programmable and only offers a small number of QoS classes. Diverse applications and evolving protocols
require a programmable data plane. For example, video traffic of gold subscribers 
should go through Unequal Error Protection (UEP) block, sensors with limited 
computation capability should use repetition coding instead of Turbo coding. 
Third, base stations are provisioned for peak demands and the excess capacity 
is idle at other points. Since the time of peak demand varies 
%erran: by base station,
from base station to base station
the network itself is heavily over-provisioned. 
Fourth, the need for RAN sharing among operators should not compromise the
ability to control the data plane. Current LTE RAN sharing mechanisms do not
provide operators direct control of the shared radio resources. 

To overcome these problems, we present PRAN, a Programmable Radio Access Network.
PRAN centralize the L1/L2 processing of base stations into a cluster of commodity
servers. 
We make the following contributions in designing PRAN: 
\begin{itemize}[leftmargin=*]
\itemsep0pt \parskip0pt \parsep0pt
\item PRAN coordinates radio resource (time, frequency, location) usage among operators 
and enables each operator to flexibly control its radio resources.
\item PRAN offers unprecedented programmability to configure different data
  paths for each operator or each application,
so that protocols can be easily upgraded and replaced.
\item PRAN enables dynamic computational resources sharing and scheduling among base stations'
processing so as to increase the utilization of the infrastructure.
\item Based on real traces of more than 3000 base stations from a national cellular operator, 
our initial evaluation shows PRAN reduces CPU requirements by a factor of 30.
\end{itemize}


